Plant regulation of essential and non-essential heavy metals.
Along two traverses near Mineral, Virginia (USA) (where soil metal concentrations vary greatly because of the natural weathering of sulphide mineral deposits), the concentrations of Cu, Zn, Cd and Pb were lower in leaves than in twigs, and the concentrations of Cd and Pb increased with twig age. Quercus alba L. and the hybridized Quercus rubra L./falcata Michx./velutina L. group contained the highest Cd and lowest Pb concentrations, whereas Cornus florida L. contained the lowest Cd and highest Pb. Evidence of selective uptake of Cu, relative to Ag, occurred in Q. alba, since (a) Cu/Ag ratios in twigs were the inverse of those in soils, (b) the Cu/Ag ratios were greater in twigs than in soils, and (c) Cu and Ag concentrations were not correlated within the twigs. On the other hand, there was evidence of nonselectivity for Cd, compared with Zn, in both Q. alba and Q. rubra/falcata/velutina, since Zn and Cd concentrations were correlated within several organs and between different organs. Other species studied showed greater selectivity in their uptake and distribution of Cd compared with Zn.